Added support for Linux on power

[ujjwalsh]

Hi,
I had added ppc64le(Linux on Power) support on travis-ci in the branch and looks like its been successfully added. I believe it is ready for the final review and merge. The travis ci build logs can be verified from the link below.

https://travis-ci.com/github/ujjwalsh/btree
https://travis-ci.com/github/ujjwalsh/btree/builds/191584661
Please have a look.

Regards,
ujjwal

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[gconnell]

Closed as we now use github actions rather than Travis.